Summary. Loss of mutual solvents during acidizing can be severe, depending on the type of product. Results showed that some mutual solvents can penetrate deeply into a test formation. Chlorination of mutual solvents by penetrate deeply into a test formation. Chlorination of mutual solvents by HCl was also considered and was found to be minimal when the treatments were designed properly,
Introduction The uses of mutual solvents as preflushes and in conjunction with acid have become widely accepted because of their success in preventing and breaking emulsions, sludges, and water blocks. preventing and breaking emulsions, sludges, and water blocks. The original concept of the mutual solvent was that it could effectively reduce the surface tension of acid from 72 to below 40 dynes/cm [72 to 40 mn/ml, reduce interfacial tensions to below 10 dynes/cm [10 mn/ml, and provide solvency for removing oil films before acidizing. With the success of the original solvents, used at 10 to 35 vol% in the acid, came a succession of "micellartype" mutual solvents, intended for use at 1 to 5 %. For many jobs, such as lowering surface tension, these low-concentration surfactant-mixture mutual solvents were very good. In a few cases, however, treating attempts with acid and some mutual solvents used to remove deep damage proved unsuccessful. Although many factors, including fluid placement, compound the difficulty of removing deep damage, it was felt that in numerous cases the acid was reaching the damage but not removing it. A set of tests was designed to duplicate the fluid/rock contact area of the deep-damage problem.
The tests involved flowing the treating fluid through a Berea sand-stone core 2 in. [5.08 cm] in diameter and 6 ft [1.8m] long. Berea was selected to provide a homogeneous matrix with representative formation clays. By encasing the core in a fiberglass wrap and using 6-in. [ 1.59-mm] -ID sample tubes to thieve fluid from the interior of the core, we could remove samples of the treating fluid from the core at set intervals and check the fluids for signs of degradation. In the laboratory tests, the surface tension of the acid/mutual-solvent system was often higher in samples taken at the farther sample collection taps immediately after acid breakthrough. This indicates that the concentration of some mutual solvents in the acid was reduced during injection of the acid/mutual-solvent mixture through the sandstone core. Although dilution of the mutualsolvent/acid volume with water from the pore spaces was considered, some of the mutual solvents seemed to be more affected than others. Because the scope of the research was limited to testing for the absence or presence of the mutual solvent through the surface tension level of the treating fluid, the identity of the loss mechanism was not established. Limited testing of other sandstones (Banders, Tubb. and Springer) with higher clay content or a different variety of clay has shown more rapid loss of mutual solvent in a few test cases. For the purposes of this paper, the loss of the mutual solvent within the Berea sandstone will be labeled with the all-encompassing term of adsorption. concern for the amount of chlorination of mutual solvents and other organic compounds by HCl accompanies the increasing use of these materials with HCl. The concern is well founded because a high concentration of some chlorinated organic materials in crude oil can result in damage to refinery catalysts. In some instances, special tests for chlorinated material in the crude oil stream have been devised.
Because the only remotely related study on chlorination dealt solely with the reduction of HCl strength by heating with primary alcohol solutions, 10 a more detailed report on mutual solvents was needed. In our tests on attempting chlorination of methyl alcohol, we could not duplicate the data reported in this earlier paper. Our analysis of the methyl-alcohol/HCl products indicated a level of chlorination about even with background readings for methyl alcohol or HCl. Finally, a study of the seriousness of the transfer of the chlorinated hydrocarbon from the acid to the oil was of prime importance.
Discussion Tests for Loss of Product. The mutual solvents tested included an alcohol-mixture mutual solvent. ethylene glycol monobutyl ether (EGMBE), and two surfactant-mixture mutual solvents. The alcohol-mixture mutual solvent is a blend of 83% isopropyl alcohol and 17% isooctyl alcohol that is miscible (a single-phase solution) at 35 vol % in a 15 % HCl solution. 4 The EGMBE is miscible in 15 % HCl at concentrations of less than I to more than 30% but is normally used at I 0 vol%. The surfactant-mixture mutual solvents are used at 1 to 5% in acid.
Undoubtedly, the amount of mutual solvent initially present will affect the outcome of this type of test. High concentrations of the solvents or mutual solvents are often needed where large amounts of oil, sludge, or emulsions are present. However, not all mutual solvents can be used at high concentrations. Many of the surfactant-mixture mutual solvents can form emulsions at concentrations 15% or greater. Even EGMBE, when used at very high concentrations, may oil-wet the formation or promote emulsions. In one well in a south Texas field, a reduction of 80% of total fluid production was observed after an acid treatment in which 35 % EGMBE production was observed after an acid treatment in which 35 % EGMBE was used mistakenly in the acid. In this field, both 10% EGMBE and the 35% alcohol mixture had been used successfully. The drawback to the high concentrations of mutual solvents, of course, is cost. The total mutual-solvent cost depends on both unit cost and the amount used.
The surfactant-mixture mutual solvents, also referred to as "micellar acting" or "stable dispersions," may contain small quantities of glycol ether products, long-chain alcohols, and several sur-factants. These products were included in our testing because other cost appeal. Some commercially available demulsifying and surface-tension-lowering surfactants were also tested. Table 1 shows the initial acid strengths, surface interfacial tensions (IFT's), and some IFT data for the acid/mutual-solvent and surfactant/acid solutions. The initial acid strengths of the mixtures reflect the actual strength of HCl in the total solution after mixing.
The adsorption tests were carried out with Berea sandstone cores 2 in. [5.08 cm] in diameter and 6 ft f 1. 8 ml long - These cores were wrapped with a triple-thickness fiberglass mat and resin as confinement.
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